1. Field of the Invention
The present invention relates to an optical amplifying apparatus to be employed in an optical repeater station of an optical communication system and, more particularly, to an optical amplifying apparatus and a wide-band optical amplifying apparatus for realizing a wider input dynamic range and a lower noise. The invention further relates to anoptical communication system including those apparatus.
At present, an optical transmission system with a super-long distance and a high capacity has been demanded for constructing the future multimedia network. As a system for realizing that high capacity, a wavelength-division multiplexing (as will be abbreviated into the xe2x80x9cWDMxe2x80x9d) system has been investigated and developed because of an advantage that it can utilize the wide band and the high capacity of optical fibers.
Especially the long-distant optical communication system is required to have an optical amplifying apparatus for amplifying the WDM optical signals, because these WDM optical signals are attenuated while being transmitted in the super-long distant optical fiber. This optical amplifying apparatus is demanded to achieve a wide input dynamic range and a low noise for the super-long distance transmissions because the distances and transmission losses are different between optical repeater stations.
2. Description of the Related Art
In the optical communication system for transmitting the WDM optical signals, the transmission distance is restricted by the gain gradient based on the gain-wavelength characteristics of the optical amplifying apparatus, if this optical amplifying apparatus is inserted into the optical transmission line. The light is amplified by a semiconductor laser or a rare earth element doped optical fiber amplifier, the gain characteristics of which have a wavelength dependency.
If the optical amplifying apparatus are connected in tandem for elongating the transmission distance, therefore, the gain gradients to be made in the individual optical amplifying apparatus are accumulated so that the optical signal-to-noise ratio (as will be abbreviated into the xe2x80x9coptical-SNRxe2x80x9d) is lowered in the channel of a lower optical level of the WDM optical signal whereas the waveforms are degraded by the nonlinear optical effects or the like in the channel of a higher optical level of the WDM optical signals.
On the other hand, the system gain of the optical communication system is different at the individual optical repeater stations because these optical repeater stations have different distances. The system gain is the difference in the optical levels to be transmitted/received between the optical repeater stations, i.e., the maximum loss value to be obtained between the optical repeater stations and, more specifically, is the value which is calculated by adding a margin to the transmission loss between the optical repeater stations. The transmission loss is dependent not only upon the distance between the optical repeater stations but also upon the temperature fluctuation or the aging of the optical transmission line.
Where the optical amplifying apparatus is to be employed for the optical repeater station, it is demanded to have a wide input dynamic range for matching various system gains. A method for matching the various system gains is exemplified by inputting input light, after this level was lowered by an optical attenuator, to the optical amplifying apparatus. In this method, the fluctuation of the system gain is compensated by the optical attenuator. Where this optical attenuator is not utilized, the input dynamic range of the optical amplifying apparatus has to be wider than the difference between the minimum system gain and the maximum system gain of the optical communication system.
Moreover, it is known that the optical signals to be transmitted through the optical transmission line such as the optical fibers are distorted by the nonlinear optical effects such as the self-phase modulations, the four-wave mixing or the cross phase modulations. These nonlinear optical effects are raised as the optical level of the optical signals to be inputted to the transmission line is raised, so that the optical level of the optical signals to be inputted to the optical transmission line is restricted. The degree of the nonlinear optical effects are different according to the kinds of optical fibers such as the dispersion shift optical fibers (as will be abbreviated into the xe2x80x9cDSFxe2x80x9d), the non-zero dispersion shift fibers (as will be abbreviated into the xe2x80x9cNZ-DSFxe2x80x9d) or the single-mode optical fibers (as will be abbreviated into the xe2x80x9cSMFxe2x80x9d), so that the upper limits of the optical levels of the optical signals are also different. of the SMF, the NZ-DSF and the DSF, for example, the SMF has the largest upper limit, and the DSF has the smallest upper limit. This difference in the upper limits is as large as several dB. In order that a single optical amplifying apparatus may cover those kinds of optical fibers, the optical attenuator is connected in the prior art with the output terminals of the optical amplifying apparatus.
In order to ensure the safety of the operator, moreover, the optical amplifying apparatus has an output terminal opening detecting function. This output terminal opening detecting function is to detect whether or not the output terminals of the optical amplifying apparatus are opened, in terms of a reflection light reflected from the open terminals, and to lower the optical level of the output light of the optical amplifying apparatus if the terminals are opened.
Here, the optical amplifying apparatus may be degraded in its noise figure if it covers a wider input dynamic range, because the output light level generally has an upper limit. Where the optical attenuator is utilized, as described above, for covering the wider input dynamic range, the optical amplifying apparatus is designed to match the maximum system gain and is used by lowering the input light level with the optical attenuator. As a result, the optical amplifying apparatus is employed with its optical-SNR being degraded, so that its transmission distance is reduced.
Moreover, the optical amplifying apparatus has to be provided with a variety of output light levels so that it may be able to be connected with many kinds of optical fibers laid already. If this necessity for the connections with the existing various optical fibers is to be satisfied by the optical attenuator, whether or not the output terminals of the optical amplifying apparatus are opened has to be detected not only at the output terminals of the optical amplifying apparatus but also at the output terminals of the optical attenuator. This m makes it necessary to detect the weak reflection light and makes it difficult to detect the opening of the output terminals.
An object of the invention is to provide an optical amplifying apparatus with little degradation in noise figure but a wide input dynamic range.
Another object of the invention is to provide an optical amplifying apparatus having various output light levels so that it can be connected with many kinds of laid optical fibers.
Another object of the invention is to provide an optical amplifying apparatus which can be connected with many kinds of optical fibers without degrading the output terminal opening detecting function.
Another object of the invention is to provide an optical amplifying apparatus capable of amplifying light having a wider bandwidth.
Still another object of the invention is to provide an optical communication system in which an optical amplifying apparatus having a wide input dynamic range without a degradation in noise figure is employed as an optical repeater station or the like.
The above-specified objects are achieved by an optical amplifying apparatus which comprises a first optical amplifying part, an optical attenuating part and a second optical amplifying part connected in tandem and a controlling part, wherein the controlling part controls the attenuation amount of the optical attenuating part in accordance with a difference of the target values of output light level corresponding to the variation amount of the input light of the first optical amplifying part or in accordance with a difference of the target values of the output light level corresponding to the variation amount of the output light of the second optical amplifying part.
Moreover, the above-specified objects are achieved by a wide-band optical amplifying apparatus which wavelength-divides input light over a wide-wavelength band and amplifies the input light in every divided wavelength band by the above optical amplifying apparatus.
Moreover, the objects are achieved by an optical communication system comprising the optical amplifying apparatus or the wide-band optical amplifying apparatus in at least one of an optical sending station, an optical repeater station and an optical receiving station.
For example, when the target value of output light level of the first optical amplifying part is changed, the controlling part changes the attenuation amount of the optical attenuating part in accordance with a difference between the target value and the changed target value. On the other hand, when the target value of the output light level of the second optical amplifying part is changed, the controlling part changes the attenuation amount of the optical attenuating part in accordance with a difference between the target value and the changed target value. Moreover, the predetermined value of the input light level is a value within one of a plurality of divided ranges of input light level and each target value of the output light level of the first optical amplifying part is respectively set in every divided range.
The optical amplifying apparatus of the invention can widen the input dynamic range without a degradation in noise figure. Moreover, a monitor circuit for detecting the input light level also covers a wide. dynamic range. The optical amplifying apparatus of the invention can be connected with many kinds of optical fibers without degrading the output terminal opening detecting function. The optical amplifying apparatus of the invention can amplify light having a wide wavelength band since it amplifies a plurality of wavelength bands. The optical amplifying apparatus can adapt to various kinds of optical transmission lines. This leads to effectively exploiting laid optical transmission lines. In addition, the optical amplifying apparatus can transmit in longer distance, resulting in reducing the number of optical repeater stations.